He’s great as far as I’m concerned, with the right upgrades. With some of his damage options, you can basically get free kills on red enemies on top of fueling the other characters’ attacks faster. Until he gets those, however, his usefulness is more situational since you’re wasting a red on getting something else without progressing the fight.
If you mean unlocking him, the game won’t offer you new party members if you already have four. Maybe it does when one dies along the way, though; I’ve only had full wipes so far ^^’